In order to assess the UV sensitivity of the PSPC, both
laboratory and in orbit calibration measurements have been taken.
UV signals in the PSPC detector are produced by only one primary
electron, and therefore UV spectra have a characteristic extremely
soft pulse height response.
The standard celestial UV source is the nearby bright A-type star Vega
(spectral type A0V), which was looked at with a deep PSPC observation.
From the laboratory calibration measurements and the known atmospheric
properties of Vega a count rate of cts/s was
expected.
The measurements yielded a weak signal only visible below
pulse height channels 15 of cts/s, i.e., ,
completely consistent with a signal caused only by UV events;
incidentally, in the all sky survey Vega was not detected.
Note in this context that the discrepancy between expectation
and measurements is still within the errors and should therefore
not be taken seriously.
Since Vega is one of the brightest UV sources in the sky,
it is probably the only PSPC source due to UV signals.
In the case of O-type stars the apparent UV flux can be similar,
but the recorded PSPC flux will always be dominated by X-ray flux.